The present invention relates to a time-of-day coincidence system and, more particularly, to a system for coinciding time of day of a plurality of information processing apparatuses each having a time of day resetting, respectively function effected in response to common clock pulses.
In a system constituted by a plurality of information processing apparatuses, in order to control a processing sequence of the apparatuses and analyze processing results in accordance with the sequence order, a time of day resetting, respectively function is provided to each apparatus. Processing utilizing the contents of display performed by the time of day resetting, respectively function, or recording of displayed time of day values during execution of processing has been frequently performed.
In a system wherein a common data file is used in a plurality of information processing apparatuses in order to improve system reliability by means of redundancy or to realize load dispersion system, processing results of each information processing apparatus are recorded together with corresponding time of Day information during processing in a corresponding journal file. If a failure should occur, the contents of the journal files are combined with reference to time of day, respectively values, thereby restoring a normal system operation. In this case, if time of day, respectively values of the respective information processing apparatuses do not coincide with each other, the execution sequence of the common data file cannot be correctly established. The system may not be restored in the normal mode.
In order to solve the above problem, the time of day respectively values of the respective information processing apparatuses must coincide with each other so as not to adversely affect control and analysis.
A method of coinciding time of day values of the respective information processing apparatuses is to cause the plurality of information processing apparatuses to use a common TOD time of day (to be referred to hereinafter as a TOD) U.S. Pat. No. 4,388,688 (June 14, 1983) discloses a tightly coupled multi-processor in which a plurality of central processing units (CPU) commonly use a TOD timer arranged in a system controller and a time of day value is read out if time of day information is necessary. This method can be applied to a loosely coupled multi-processor. However, if there are variations in timing for reading out time of day information from the common time of day timer, TOD errors occur in the plurality of CPUs.
In order to shorten the TOD timer read access time, all bits of the time of day information can be output as parallel output bits. However, if the number of information processing apparatuses is increased, a large number of signal lines are undesirably required.
In the loosely coupled multi-processor, since a common TOD timer cannot be arranged in an interface for "tightly" connecting with all the processors, the time for executing a time of day read sequence is not negligible, and, accordingly, variations in time of day values of the respective information processing apparatuses are increased.
In addition, according to the above method, each information processing apparatus must read out time of day information whenever time of day information is needed, resulting in that, when the number of information processing apparatuses is increased, TOD timer read access requests are frequently generated. For this reason, lines and a control circuit for transfer of the readout data are complicated and expensive. If sufficient hardware for read access control and data transfer control is not provided, waiting time for read access requests is increased due to read contention, thereby degrading performance of each information processing apparatus.
In order to solvethe above problem, the following implementations are required:
(1) A counter which stores the latest time of day information is arranged in each information processing apparatus;
(2) The common TOD timer count must be set in each counter at the start of processing; and
(3) The counts of the counters must be incremented so as not to cause a time lag between the information processing apparatuses.
With the above implementations, since the latest time of day information is present in each information processing apparatus, the common TOD timer need not be accessed whenever time of day information is needed. Therefore, even if the number of information processing apparatuses is increased, read contention does not frequently occur.
With the above implementations, in order to improve TOD coincidence precision, the following requirements must be satisfied:
(1) A set value in each counter has a minimum deviation from the value of the common TOD timer; and
(2) Variations in counts of the counters in the respective information processing apparatuses must be minimized after the common initial value is set in each counter.
In order to satisfy requirement (1), hardware becomes bulky since time of day signal lines must be arranged to read out data from the common TOD timer and all bits of the time of day information are transferred as parallel output bits.